The present invention relates to a positioner for a fluid, particularly pneumatic, actuator driven by pressurising fluid, as often finds application for actuating translational or rotational final control elements, such as valves, in process and system engineering. A positioner for a fluid actuator has an electronics assembly which via a positional setpoint/positional actual value comparison outputs a signal for feedback control of a current/pressure transducer for controlling the fluid actuator. The current/pressure transducer may comprise a pneumatic pilot stage via which the pneumatic main stage connected to a particularly constant compressed air source may be activated, the main stage driving a final control element of the actuator by pressurising fluid activation. As an alternative the current/pressure transducer may be configured as a structural unit.
Published German Application DE 44 29 401 C2 discloses such a positioner for which an inherently safe quick-action ventilating system according to type “e” protection is proposed which is engineered to run the final control element to a fail-safe position when called for. In this known positioner, a feedback control electronics assembly and the current/pressure transducer are powered by an analog power supply defined by 4 to 20 mA and 9.6 V. The electronics assembly is powered directly by the analog power supply. The current/pressure transducer is powered via an electrical conductor from the electronics assembly with both a feedback control signal and with the power of the analog power supply as is non-consumed by the electronics assembly. When the system is down or at fault, the control element of the actuator is run to a fail-safe position by the actuator being dumped. This is achieved by an emergency switch having a spring for automatically open-circuiting the emergency switch in the electrical conductor between the electronics assembly and current/pressure transducer. The emergency switch also has a coil connected to a usually binary emergency voltage input of 12 V via which the coil provides electromagnetic forces in closing the emergency switch by overcoming the spring force. When the voltage is missing—in other words, when the system is down—the emergency switch is open-circuited to break the electrical connection between the electronics assembly and the current/pressure transducer, resulting in the current/pressure transducer for dumping the actuator being out of circuit.
The structure of this known positioner is risky in terms of safety in that there is no total separation of the electric circuits of the positioner from its analog power supply and of the dump system from its binary emergency voltage. It has been discovered, namely, that a “residual current”—despite the system being down, in other words with lack of the 24 V emergency voltage—flows in the electrical conductor between the electronics assembly and the current/pressure transducer, inducing stray currents in the coil of the switch. The closing forces induced by these stray currents in the emergency switch may even be sufficient to maintain the emergency switch closed for a time which is at least still risky for safe OFF and during which the actuator cannot be dumped to the fail-safe position instantly when the system is at fault. Furthermore, because of its mechanical spring component, the emergency switch structure may malfunction because of material fatigue, risking the current/pressure transducer not being signalled OFF with the mandatory high assurance. In conclusion, the power supply balance of this known positioner has disadvantages in that, because of having to operate the current/pressure transducer by the analog power supply, only a restricted amount of energy of the analog power supply is available for consumption by the electronics components of the positioner.